This invention pertains to means for providing protection from transient voltage surges induced in coaxial cables and the like which carry television signals from antenna systems or cable television distribution services. Although television antennas are usually provided with lightning arresters or antenna discharge devices, and cable systems have protection devices for the distribution amplifiers, there is insufficient protection for the television receivers and/or cable-converters at the indoor site where such equipment is typically located. Reports from high lightning areas have indicated that more than fifty percent of the households served by a single cable-television service have experienced significant equipment damage during a particularly severe electrical storm.
The problem of protecting home television equipment from transient voltage surges caused by lightning is compounded because such surges can enter the equipment through either its connection to the household power distribution system or through the coaxial cable system supplying the television connections simultaneously. Television equipment is most commonly connected to the household power line by means of a "two-wire" line cord. The "neutral" or low-potential wire is grounded to earth by a connection made at the electrical utility service entrance to the building. This wire is commonly referred to as the "grounded" conductor. Some television equipment such as video cassette recorders (VCRs) and cable-converters employ three-wire line cords which connect exposed metallic enclosures to an additional third terminal frequently provided at the wall outlet receptacle. This third or ground terminal is connected to what is commonly referred to as a "grounding" conductor. This grounding conductor is a separate wire and/or metallic conduit that is also connected to earth at the building electrical utility service entrance.
The coaxial cable may enter the premises from either overhead or underground. The outer conductor or shield of the cable is grounded to earth at some outdoor location, such as at the utility pole or underground terminal from which a cable television feeder branch is connected to serve the premises, or at an antenna discharge device (lightning arrester) connected to an outdoor earth grounding rod. In spite of these ground connections, nearby lightning strokes can induce large transient voltages in the coaxial cable, whether it is overhead or underground. Such voltage surges will appear at the input to the television equipment to which it connected.
This surge voltage will "seek earth" through the television equipment. If there is insufficient threshold breakdown voltage between the cable input of the television equipment and the neutral or grounding conductor of the power cord there will occur an arc-over, and destructive currents may flow through the equipment to the earth connection at the utility service entrance of the building.
In a similar manner, transient voltage surges induced in the power distribution system will propagate throughout the premises and may appear at every electrical outlet. These voltages will also "seek earth", and may cause destructive currents to flow through the television equipment and the coaxial cable to its remote earth connection.
The possible flow of destructive currents from lightning-induced transient voltage from two sources to the television equipment explains why using only a power line surge protector by itself or only a coaxial cable surge protector by itself has often failed to provide the needed protection.
Proceeding according to the above description of the transient voltage surge environment involving coaxial cable connected television equipment, it is known in the art that adequate protection can best be provided by combining the coaxial cable protector with the power line protector in a single protection unit. Such protection units are typically in the form of a wall-mountable housing having a male three-pin wall plug rigidly affixed to the back surface thereof for insertion into a three-conductor power outlet receptacle on the wall. Two of these plug pins provide power from electric power lines, and the third pin is thereby connected to the "grounding" conductor of the power outlet receptacle which leads to the power system local ground. The power line protector is incorporated into the protection unit and is connected to transfer power from the power input pins of the protection unit plug to the corresponding pins of an appliance socket mounted on the housing. The power line surge suppressor is grounded to the grounding pin of the protection unit plug. The coaxial cable protector is provided within the protection unit, having input and output connectors extending therefrom. The coaxial cable bringing in cable television signals is connected to the input of the coaxial cable protector, and a length of coaxial cable is connected between the cable protector output and the television equipment signal input terminal. The coaxial cable protector has a ground lead which connects the outer connectors of the coaxial cable protector to the grounding pin of the power input plug, and a ground connection is similarly provided between one or more appropriate elements of the coaxial cable surge protector and the above-mentioned grounding pin. All necessary grounds are thus returned to a common point over very short leads.
The performance requirements for television coaxial cable surge suppressors are severe. Television cable system operators often supply service to a large geographical area serving thousands of subscribers. They must assure an adequate signal level at the subscriber's site to guarantee good picture quality. Because of the losses inherent in the coaxial cables used to distribute the signals, distribution amplifiers are employed at various locations throughout the system to restore the signals to adequate levels. Line taps to individual premises also introduce losses, as do lengthy cable runs from utility poles or underground terminals to the subscriber's television equipment location in the residence. The economics of the distribution system usually dictate that as small a margin as is practicable to assure good picture quality be provided in the level of signal at the subscriber's site. Often this margin may be so small as to permit no significant further attenuation of the signal by the coaxial cable surge suppressor. Some cable operators demand that the suppressor introduce no more than one decibel of insertion loss over the entire frequency range. One-half decibel, or less, insertion loss would be preferred if achievable.
In addition to the low attenuation or insertion loss requirement of a coaxial cable surge suppressor, it should be capable of suppressing the transient voltages to very low levels. Transient voltage surges on coaxial television cables can reach magnitudes of the order of three thousand volts before there is arc-over in the cable or in the cable connector plugs and receptacles used in the system. To protect sensitive tuner input circuits and microprocessors employed in modern television receiving equipment, the surge should be suppressed to the order of several volts.
The prior art has recognized this need to provide means for suppressing surges at the coaxial cable input to household television equipment. Some of the prior art devices are not combined with power line surge suppressors, although they do provide a low impedance connection to the grounding conductor at an electrical outlet receptacle. Other prior art devices do combine coaxial cable surge suppression with power line surge suppression. However, all such prior art devices have been found to have one or more of the following deficiencies in the performance or construction of the coaxial cable surge suppressor:
1. The coaxial cable suppressor is not combined with a power line surge suppressor, or if it is, it may not share a common low impedance connection to the grounding conductor of the power line receptacle.
2. The coaxial cable suppressor employs surge suppression components which are more applicable to power lines or communication lines such as metal-oxide-varistors (MOV's) or silicon avalanche diodes. Such devices have fairly high capacitance and present a low reactance at the high frequencies of the television signals. This causes unacceptable attenuation of the television signal.
3. Some prior art devices employ only a single gas discharge device shunted across the coaxial cable from center conductor to the outer conductor (shield) of the cable. While such devices may be of sufficiently low reactance to avoid excessive signal attenuation, their slow response to rapidly rising transient voltages may result in a let-through voltage of hundreds of volts prior to the device striking and suppressing the surge. Also, low level transients may not cause the device to strike, thus permitting transient voltages of several hundred volts to be passed through to the television equipment. This voltage may be high enough to damage sensitive input circuits or other components in the equipment.
The present invention provides a solution of these problems.